The controller of the creature chooses which attacking creature that creature blocks.

If you bring a card into the game from outside the game in a Sealed Deck or Booster Draft event, you may select only a card that is in the card pool that you built your deck from.

If you bring a card into the game from outside the game in a Constructed tournament (or a Constructed game played under tournament conditions), you may select only a card from your sideboard. In casual Constructed games, the card comes from your collection.

You choose whether you want the creature to gain reach until end of turn as Academic Dispute resolves.

If a card is brought into the game from outside the game, it will stay in the game until it ends or until its owner leaves the game, whichever comes first.

If instructed to learn, you may do nothing. Discarding a card and putting a Lesson card into your hand are both optional.

The creature blocks only if it's able to do so as the declare blockers step begins. If, at that time, the creature is tapped, it's affected by a spell or ability that says it can't block, or no creatures are attacking its controller or a planeswalker controlled by that player, then it doesn't block. If there's a cost associated with having the creature block, the player isn't forced to pay that cost. If that cost isn't paid, the creature won't block.